1. Field of the Invention
The present invention relates to a motor drive circuit that controls rotation of a motor.
2. Description of the Related Art
Brushless direct current (DC) motors are used for rotating cooling fans or disk-type media. A brushless DC motor generally comprises a rotor provided with a permanent magnet and a stator provided with multiple phase coils connected together in a star connection. The coil is excited by controlling a current supplied thereto such that the rotor is driven by relatively rotating the rotor relative to the stator. The brushless DC motor generally comprises a sensor such as a Hall element or an optical encoder for detecting a rotational position of the rotor, and switches a current to be supplied to each phase coil in accordance with the position detected by the sensor, so that the rotor is supplied with an adequate torque.
A method (hereinafter, referred to as a “sine wave drive method”) is known in which, when providing 180-degree excitation of a three-phase motor, a switching voltage to be supplied to each phase coil is pulse-modulated such that each phase coil current is brought close to a sine wave. A drive noise from the motor can be reduced by the sine wave drive.